The unused 90% of the brain from a scientific standpoint

What are the theories behind the unused parts of the brain?

Given the fundamental theories behind evolution, it's unlikely such an inefficient waste would have developed and still be maintained after time.

That 90% doesn't actually exist. It's based on old research that got twisted and distorted into a myth. We use all of our brains, just not at once. Information is shuttled around from one area to another, depending on the information and the task at hand. There's really no way to increase activity without either increasing the size and complexity of the brain, or having the different signals running into each other. A good example is a mechanical pocket watch. Not all the gears spin at top speed, and not all of them are moving at once. If you were to boost their speed and run them nonstop, the watch would break (or at least cease to keep accurate time). In neurological terms, it would translate to a massive seizure.

You're right in your observation about evolution, though. If the brain were that inefficient, evolution would trim the unnecessary parts away (especially since the brain accounts for so much of our body's energy use as it is). The premise is incorrect, though - there are no unnecessary parts of the brain to eliminate.

This illustrates a point about how science works. You failed to do the first step, which is to gather data/knowledge about a phenomenon that you are going to study. Because of this, you failed at step 2 which is to make a hypothesis.

Statement to test: Natural Selection would not allow such an inefficient organ to develop.

How are you going to test this hypothesis? Are you going to cut out pieces of brain and see if people have any changes in function? Are you going to study cases of traumatic brain injury to see where people have lost specific parts of their brain and see if there were any consequences? Without testing, your hypothesis is worthless.

To put is more abstractly:

You made a statement A.
A should not be true because of theory X.
Therefore, there is a problem. Either theory X or statement A is wrong.

In this case, because of lack of training in neuroscience, your statement A is wrong.

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Thus, by your own reasoning, don't you think it's possible that the whole notion of so-called "unused brain" is in fact, not true to begin with? Note: this is a rhetorical question.

The conventional wisdom that we only use 10% of our brain is a myth.
Humans use 100% of their brain - not all at once to be sure, but 100% none the less.